#define _CRT_SECURE_NO_WARNINGS 1
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == nullptr || head->next == nullptr) return head;

        ListNode* newhead = head->next;
        head->next = swapPairs(head->next->next);
        newhead->next = head;
        return newhead;
    }
};